1. Field of the Invention
This invention relates to surface modified thin porous membranes, modified with a perfluorocarbon polymeric composition and to a process for making the membrane.
2. Description of the Prior Art
Porous membrane filters are utilized in a wide variety of environments to separate materials within a fluid stream. Membranes are formed from a solid polymeric matrix and have highly precisely controlled and measurable porosity, pore size and thickness. In use, the membrane filters generally are incorporated into a device such as a cartridge which, in turn, is adapted to be inserted within a fluid stream to effect removal of particles, microorganisms or a solute from liquids and gases.
To be useful, membrane filters must be resistant to the fluid being filtered so that it maintains its strength, porosity, chemical integrity and cleanliness. For example, in the manufacture of microelectronic circuits, membrane filters are used extensively to purify various process fluids to prevent contaminants from causing circuit failures. Fluid filtration or purification is usually carried out by passing the process fluid through the membrane filter under a differential pressure across the membrane which creates a zone of higher pressure on the upstream side of the membrane than on the downstream side. Thus, liquids being filtered in this fashion experience a pressure drop across the membrane filter. This pressure differential also results in the liquid on the upstream side having a higher level of dissolved gases than the liquid on the downstream side. This occurs because gases, such as air, have greater solubility in liquids at higher pressures than in fluids at lower pressures. As the liquid passes from the upstream side of the membrane filter to the downstream side, dissolved gases come out of solution in the membrane resulting in outgassing of the liquid. Outgassing of a liquid can also occur spontaneously without a pressure differential as long as the liquid contains dissolved gases and there is a driving force for the gases to come out of solution, such as nucleating sites on the surfaces of a membrane where gas pockets can form and grow. Outgassing liquids typically used in the manufacture of semiconductors and microelectronic devices usually include very high purity water, ozonated water, organic solvents such as alcohols, and others which are generally significantly chemically active, such as concentrated and aqueous acids or bases which can contain an oxidizer. These chemically active liquids require the use of a chemically inert filter to prevent membrane degradation. Membrane degradation leading to the chemical breakdown of the membrane composition usually results in extractable material which is released from the filter during use, thus compromising the purity, integrity and cleanliness of the fluid being filtered. Fluorocarbon-based membrane filters made from fluorine-containing polymers such as polytetrafluoroethylene are commonly utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness, or excellent resistance to chemical attack. One disadvantage of fluorine-containing polymers is that they are hydrophobic and therefore membranes made from such polymers are difficult to wet with aqueous fluids or other fluids which have surface tensions greater than the surface energy of the membrane. Another problem often encountered during the filtration of outgassing liquids with a hydrophobic membrane filter is that the membrane provides nucleating sites for dissolved gases to come out of solution under the driving force of the pressure differential, during the filtration process. Gases which come out of solution at these nucleating sites on the hydrophobic membrane surfaces, including the interior pore surfaces and the exterior or geometric surfaces, form gas pockets which adhere to the membrane. As these gas pockets grow in size due to continued outgassing, they begin to displace liquid from the pores of the membrane ultimately reducing the effective filtration area of the membrane. This phenomenon is usually referred to as dewetting of the membrane filter since the fluid-wetted, or fluid-filled portions of the membrane are gradually converted into fluid-nonwetted, or gas-filled portions. Dewetting of a membrane can also occur spontaneously when a wet membrane, such as a hydrophobic membrane wet with an aqueous fluid, is exposed to a gas such as air. It has been found that this dewetting phenomenon occurs more frequently and is more pronounced in fluorocarbon-based membranes made from fluorine-containing polymers such as polytetrafluoroethylene. It has also been found that the rate at which dewetting occurs is greater in small pore size membranes such as 0.2 microns or less, than in larger pore size membranes. During a filtration process the reduction of effective membrane area available for filtration due to dewetting of the membrane in a filter device results in a reduction of the overall filtration efficiency of the filter. This reduced efficiency manifests itself in a reduction in liquid flow rate through the filter at a given pressure drop or in an increase in pressure drop at a given flow rate. Thus, as the membrane filter dewets with time, the user is not able to purify or filter the same volume of process liquid per unit time as when the filter was newly installed and therefore completely wet. This reduction of the overall throughput capability of the filtration process results in an increase in the user's time and cost to purify a unit volume of process liquid. Faced with a throughput reduction, the user is often required to install new filters in the process and to discard the dewet filters. This premature filter changeout due to dewetting and not necessarily due to the exhaustion of the filter's dirt-holding capacity results in unscheduled downtime and increases the user's overall cost. Optionally, the user can compensate for the reduction in efficiency by making adjustments to other elements of the filtration system such as increasing the speed at which a pump forces liquid through the filter to increase the pressure drop across the membrane, thus maintaining a constant flow rate. These adjustments also translate into higher operating costs for the user and increases the potential for malfunction of the other elements in the system as well as the potential for a process liquid spill due to the increased processing pressures. Another option for the user to avoid premature filter changeout due to dewetting is to treat the filter to rewet the membrane. The treatment is time consuming since it requires that the filter device be removed from the filtration system resulting in unscheduled downtime and can often result in the introduction of contaminants derived from the rewetting process into the process liquid passing through the filter. Typically, a low surface tension rewetting agent may be used, including alcohols such as isopropanol, which are flammable liquids that cause safety concerns. Prior to placing the filtration device back into service, the end user rewets the dewet filter with the alcohol followed by a water flush and then a flush with the process liquid. While membrane manufacturers may have the expertise for handling and treating dewet filters, end users may not have the capabilities or the desire to perform such additional costly processing steps.
All membranes are characterized by nominal pore size which is directly related to the membrane's particle retention characteristics. Pore size is directly proportional and particle retention is inversely proportional to flow rate through the membrane. It is desirable to maximize both particle retention and flow rate. Significantly increasing one of these characteristics while signficantly reducing the other of these characteristics is undesirable.
U.S. Pat. No. 4,470,859 to Benezra et al, discloses a process for modifying the surfaces of microporous substrates formed of a fluorocarbon such as polytetrafluoroethylene, with a coating of a perfluorocarbon copolymer from a solution of the copolymer to render the surface of the membrane more water wettable. The perfluorocarbon copolymer is dissolved in a solvent at elevated temperature. The membrane then is immersed into the solution which, in turn, is placed into a vacuum chamber. The pressure within the chamber then is reduced such as to approximately 150 millimeters of mercury (absolute) to remove air from within the filter. Thereafter, the pressure within the chamber is quickly returned to atmospheric pressure. This coating process is repeated to ensure, what is described by Benezra et al., complete solution penetration into the pores of the membrane. By proceeding in this manner, the membrane surfaces and the interior walls defining the interstices within the membrane are coated with the perfluorocarbon copolymer. Following the coating step, the solvent is removed by evaporation using heat and vacuum, or the solvated perfluorocarbon copolymer is precipitated with a substance in which the copolymer is effectively insoluble. The solvents utilized to form the solution include halocarbon oil, perfluorooctanoic acid, decafluorobiphenyl, N-butylacetamide, and N, N-dimethylacetamide. Subsequent to modifying the membrane surface, Benezra et al, teaches avoiding the use of a fluid containing a solvent for the modifying copolymer on the membrane surface. Benezra et al. also disclose that alcohol solutions of the copolymer should be avoided.
U.S. Pat. Nos. 4,433,082 and 4,453,991 disclose a process for forming solutions of a perfluorinated ion exchange polymer such as copolymers of tetrafluoroethylene and methyl perfluoro (4,7-dioxa-5-methyl-8-nonenoate) or perfluoro (3,6-dioxa4-methyl-7-octene sulfonyl fluoride) utilizing solvents which are relatively innocuous as compared to the solvents utilized in the coating process set forth above. The perfluorinated ion exchange polymers are dissolved in alcoholic solvents such as isopropanol at elevated temperature and pressure. The solutions obtained are disclosed as being useful in making and repairing films and non-porous membranes used in electrolytic processes such as aqueous sodium chloride electrolysis, in coating substrates such as catalyst supports for use in promoting a wide variety of chemical reactions, for coating porous diaphragms to convert them into non-porous articles and in recovering used perfluorinated polymers having sulfonic acid or sulfonate functional groups for reuse. In electrolytic processes, such as disclosed by these patents, extractables derived from the coated diaphragms are not a substantial concern and the degree of porosity of the modified diaphragm is unimportant.
Solutions of sulfonyl fluoride-containing fluoropolymers are also disclosed in U.S. Pat. No. 4,348,310. The solvents utilized therein are completely halogenated, saturated hydrocarbons, preferably having at least one terminal sulfonyl fluoride polar group. The solutions are disclosed as being used to repair holes in membranes made from fluorinated polymers and for making ion exchange film membranes, dialysis membranes, ultrafiltration and microfiltration membranes. Another disclosed use for these solutions is to coat porous diaphragm for electrochemical cells by contacting a diaphragm with the solution followed by evaporating the halogenated solvent and then hydrolyzing the coated diaphragm to convert the sulfonyl fluoride groups to the acid or salt form.
U.S. Pat. No. 4,902,308 to Mallouk et al, also describes a process for modifying the surface of a porous, expanded polytetrafluoroethylene membrane with a perfluoro-cation exchange polymer from a solution of the polymer. Mallouk et al, also teaches that contact of the surface modified membrane with fluids containing a solvent for the polymer also should be avoided.
U.S. Pat. Nos. 4,259,226 and 4,327,010 disclose modifying a porous membrane surface with a fluorinated polymer having carboxylic acid salt groups. No process steps are disclosed for controlling extractables from the membrane or for controlling the extent of binding of the modifying composition to the membrane surface.
U.S. Pat. Nos. 5,183,545 and 5,094,895 disclose a process for making a multilayer, composite, porous diaphragm from a porous, multilayer, expanded polytetrafluoroethylene substrate having its surface modified with a perfluoro ion exchange polymer composition. The modifying polymer composition can contain a surfactant and may contain excess modifying composition, both of which are sources of undesirable extractables. In addition, these patents disclose a process for coating a thick polyfluorocarbon diaphragm having a thickness exceeding 0.25 mm, preferably between about 0.76 mm and about 5.0 mm with a perfluoro ion exchange polymer. Thin membrane substrates are specifically excluded as is the use of perfluoro ion exchange polymer coatings having an equivalent weight greater than 1000.
Accordingly, it would be desirable to provide thin porous membranes having a modified surface which improves its wettability characteristics. In addition, it would be desirable to provide such a membrane which is resistant to chemical attack, such as a porous membrane formed of a fluorine-containing polymer. Furthermore, it would be desirable to provide such a membrane which does not promote nucleation of gases on its surfaces when filtering outgassing liquids such that it does not dewet during use. Also, it would be desirable to provide such a membrane having improved particle retention characteristics as compared to an unmodified membrane without significantly adversely affecting the flux characteristics of the resulting membrane, particularly with small pore size membranes.